A secondary lithium ion battery is a rechargeable power source that can be implemented into a wide variety of stationary and portable applications. The structure and electrochemical reaction mechanism of a lithium ion battery provide it with several desirable characteristics including a relatively high energy density, a relatively low internal resistance, a general non-appearance of any memory effect when compared to other types of rechargeable batteries, for example, a nickel-cadmium battery, and a low self-discharge rate. These characteristics have made the lithium ion battery the preferred mobile power source for portable consumer electronics such as laptop computers and cell phones. Larger-scale versions that interact with a multitude of interconnected systems have also been designed and manufactured by the automotive industry in an effort to improve vehicle fuel efficiency and reduce atmospheric pollution. The powertrains of hybrid electric vehicles (HEV) and extended range electric vehicles (EREV), for example, rely on the cooperative effort of multiple lithium ion batteries, usually grouped together in a lithium ion battery pack, and a hydrocarbon-fueled internal combustion engine to generate power for vehicle operation.
A lithium ion battery generally contains one or more electrochemical battery cells that include a negative electrode, a positive electrode, and a separator sandwiched between the electrodes. Each of these battery components is wetted with a liquid electrolyte solution that can communicate lithium ions. The negative and positive electrodes are formed of different materials that can store intercalated lithium at different electrochemical potentials. These electrodes may be interruptably connected by an external circuit that provides an electrical current path around the separator. The electrical circuit, as such, communicates an electrical current between the electrodes around the separator while lithium ions migrate through the separator either spontaneously (discharge phase) or non-spontaneously (charging phase). An electrical load may be coupled to the external circuit during cell discharge to make use of the electrical current. Conversely, an applied voltage from an external power source may be coupled to the external circuit to drive the reverse the electrochemical reactions that transpired during cell discharge. Each of the negative and positive electrodes may also be intimately associated with a metallic current collector to supply and receive the electrical current to and from the external circuit depending on the operating state of the electrochemical cell.
The separator functions to provide a porous and electrically insulative mechanical support barrier between the two electrodes. The separator has to be sufficiently porous to permit the internal communication of lithium ions and, at the same time, exhibit a mechanical structure that is able to physically and electrically separate the negative and positive electrodes so that a short-circuit is prevented. An extruded or cast thin-film polyolefin membrane derived from simple low carbon number olefins, such as polyethylene and polypropylene, has conventionally been implemented as the separator because of its chemical stability, mechanical strength, and relatively low cost. Imparting a workable porosity into these types of membranes often requires uniaxial or biaxial stretching during their manufacture. For example, a “dry technique” for making a polyolefin membrane involves melting a polyolefin feedstock, extruding the melted polyolefin into a film, annealing the film, and then uniaxially stretching the film. A “wet technique” may also be practiced in which a polyolefin feedstock is first mixed with a hydrocarbon or other low-molecular weight liquid substance. The mixture is then heated, melted, extruded into a sheet, and biaxially stretched. The hydrocarbon or other low-molecular weight liquid substance is eventually extracted after stretching.
An extruded or cast thin-film polyolefin membrane formed from polyethylene and/or polypropylene, however, is potentially susceptible to certain performance declines when heated excessively. Exposure of the electrochemical battery cell to temperatures of 80° C. and above can cause the polyolefin membrane to shrink, soften, and even melt. Such high temperatures can be attributed to charging-phase heat generation, ambient atmospheric temperature, or some other source. The physical distortion of a polyolefin membrane may ultimately permit direct electrical contact between the negative and positive electrodes and cause the electrochemical cell to short-circuit. Battery thermal runaway is also a possibility if the electrodes come into direct electrical contact with one another to an appreciable extent. This tendency of an extruded or cast thin-film polyolefin membrane to lose some thermal stability at temperatures exceeding 80° C. for prolonged periods is a potential concern for some lithium ion battery applications.
A separator fabricated at least in part from one of several types of engineering polymers that exhibit better thermal stability than, and at least comparable chemical stability to, polypropylene and polyethylene could potentially enhance the temperature operating window of an electrochemical battery cell and, consequently, a lithium ion battery. But the “dry” and “wet” membrane fabrication techniques often used to make a thin-film polyolefin membrane generally cannot transform these types of polymers into a membrane that exhibits sufficient porosity across its thickness at reasonable costs. The stretching techniques often employed in conventional thin-film polyolefin membrane manufacturing processes have also been shown to adversely affect the dimensional stability of membranes formed from certain engineering polymer materials at elevated temperatures above 80° C. and, more noticeably, above 100° C. A fabrication method that can reliably produce a thermally stable and sufficiently porous separator from a robust array of polymer materials is therefore needed.